Immune evasion is a hallmark of cancer, and antibodies that block the binding of CD47 protein to the SIRPα increase clearance of cancer cells by the immune system. CD47 is ubiquitously expressed at low levels on the surface of normal cells. It typically serves as a marker of self—a “don't-eat-me” signal—such that immune cells do not engulf and kill host cells. As cells age, get damaged, or become diseased, they turn on various “eat-me” signals and lose the CD47 “don't-eat-me” signal. Macrophages, a surveillance component of the innate immune system, recognize the up-regulated “eat-me” signals and eliminate those aberrant cells. Normal cells, which generally do not express these pro-phagocytic or “eat-me” signals, are protected from this immune surveillance.
Similar to aging and damaged cells, cancerous cells also express “eat-me” signals, primarily through cell-surface calreticulin. Cell-surface calreticulin exposes the cancer cells of many solid tumors to macrophages. The binding of calreticulin (an “eat-me” signal) to its LRP1 receptor on macrophages induces phagocytosis. However, cancer cells are a special class of diseased cells that are able to inhibit and avoid phagocytosis by macrophages due of an ability to turn on and amplify the dominant CD47 “don't-eat-me” signal. The progression from normal cell to cancer cell involves changes in genes and/or gene expression that trigger programmed cell death (PCD) and programmed cell removal, the two obstacles that cancer cells must overcome to survive. Many of the required steps in cancer progression subvert the multiple mechanisms of PCD, but, to date, only one step is known to be essential to subvert programmed cell removal—the expression of the dominant “don't-eat-me” signal, CD47.
In multiple preclinical studies, CD47 has been identified as an anti-phagocytic signal that is highly expressed on all types of human primary cancers investigated thus far. CD47 may be an indispensable means by which cancer cells overcome the intrinsic expression of their “eat-me” signals that result from genetic and/or epigenetic changes. Binding of CD47 to SIRPα, an inhibitory receptor on macrophages, counteracts the “eat me” signal from calreticulin and resultantly blocks phagocytosis. The CD47-SIRPα interaction results in phosphorylation of immunoreceptor tyrosine-based inhibitory motifs (ITIMs) on the cytoplasmic tail of SIRPα and the recruitment of SHP-1 and SHP-2 phosphatases, which is thought to block phagocytosis by preventing myosin-IIA accumulation at the phagocytic synapse.
Antibodies that block the binding of CD47 to its inhibitory SIRPα receptor on macrophages increase phagocytosis of cancer cells by mouse or human macrophages in vitro. Additionally, anti-CD47 antibody-mediated phagocytosis by macrophages enables presentation of intracellular proteins as peptides by MHC-I surfaces molecules to TCRs on CD8+ T cells, resulting in activation of cytotoxic T cells. Usage of patient tumor xenotransplantation mouse models has also found that CD47-blocking antibodies can not only inhibit or eliminate primary tumor growth, but also prevent and eliminate metastasis. Interestingly, some cancer cells are able to escape phagocytosis even after anti-CD47 antibody treatment. Further, anti-CD47 antibody binding to chronic lymphocytic leukemia (CLL) cells has also been able to directly induce cell death in the absence of macrophages. Because CD47 is overexpressed in all cancer types, antibodies that block CD47 have the potential to enable the patient's immune system to identify and destroy all primary or metastatic cancer cells. However, the observed differential response of cancer cells to anti-CD47 antibody treatment has yet to be examined.